From Current Literature

To obtain a source of annihilation radiation, Klemperer made use of the phenomenon of artificial radioactivity with positron decay, observed in graphite when it is bombarded by fast protons or deuterons. A graphite plate irradiated for 15 min with protons of energy 600 keV was wrapped in a layer of metal sufficient for the complete absorption of positrons. This metal was the source of the annihilation radiation of the positrons.

The radiation source prepared in this way was placed between counters, in which the number of coincidences was observed. The rather long period of positron decay of graphite (11 min.) made it possible to carry out three successive 6-minute measurements of coincidences. During this time the intensity of the source decreased by approximately a factor of 4.

The author’s experiments showed that in this case there is an increase in the number of coincidences of counter discharges: approximately one coincidence per 200 discharges of a single counter. The small magnitude of the observed effect is due to the fact that not all $\gamma$-quanta incident on a counter produce a discharge in it. According to the author’s estimates, the observed effect has the value of the correct order of magnitude. Thus Klemperer’s experiments establish the presence of pair emission of $\gamma$-quanta in positron annihilation.

In addition, it was established that, within the limits of measurement error, no changes are observed on passing from lead, in which the graphite plate was wrapped, to aluminum.

The author also investigated the question of the existence of a hard component in annihilation radiation. For this purpose the author placed the radiation source not between the counters, but on the side of one of them. In this case coincidences of discharges could be caused only by Compton electrons from hard quanta, whereas Compton electrons from the component at 510 keV could not simultaneously enter both counters, owing to their absorption in the copper plates covering the counters. The author’s data indicate the absence of any noticeable increase in the number of coincidences in this case.

Considering all the data mentioned, Klemperer comes to the conclusion that, of the three possible annihilation processes, the most probable is the first, while the second and third, if they occur, occur extremely rarely.

L. Groshev

Literature

1. Tibaud, C. R., 197, 1629, 1933; 198, 562, 1934.
2. Joliot, C. R., 197, 1622, 1934; 198, 81, 1934.
3. Crane a. Lauritsen, Phys. Rev., 45, 430, 1934.
4. Klemperer, Proc. Cambr. Phil. Soc., 30, 347, 1934.

Excitation of X-rays by Fast Ions